A number of catalysts have been suggested to convert engine exhaust gas components like carbon monoxide (CO), hydrocarbons (HC's), and nitrogen oxides (NO.sub.x) into other gases. The first two are desirably oxidized to H.sub.2 O and CO.sub.2 while the nitrogen oxides present in the exhaust gas, generally nitric oxide, are desirably reduced to N.sub.2. These so called "three-way" catalysts achieve simultaneous efficient (conversion &gt;80%) removal of CO, HC, and NO.sub.x when the fuel mixture of an internal combustion engine is slightly "rich" in fuel, i.e., in a narrow A/F ratio range between about 14.7 and 14.4, and the exhaust gas is slightly reducing. Such three-way catalysts are not efficient, however, in the reduction of NO.sub.x when engines are operated on the lean (reduced fuel) side where the A/F ratio is greater than 14.7, generally 19-35, and the exhaust gas is richer in oxygen. It is desirable, however, to operate engines on the lean side to realize a benefit in fuel economy, estimated to be in the range of 6-10%.
It would be desirable to have a catalyst system which would be effective in reducing nitric oxide emissions and also provide high conversions for hydrocarbons and carbon monoxide under lean-burn conditions (oxygen rich exhaust situations). Such a system would allow for improved fuel economy. In lean burn situations, considerable success has been achieved in the catalytic oxidation of unburned hydrocarbons (HC's) and carbon monoxide, (CO) but the reduction of the nitrogen oxides has proven to be a much more difficult problem. This is because the reducing substance (such as HC, CO or H.sub.2) tend to react more quickly with the oxygen present in the exhaust gas than with the oxygen associated with nitrogen in NO.sub.x.
Two-stage conversion systems have been proposed for treating lean-burn exhausts and involve an initial contacting zone directed to removal of NOx and a second contacting zone directed to removal of CO and HC's. For example, Subramanian et al in U.S. Pat. No. 5,399,324, commonly assigned with this invention, disclose a catalyst system for lean burn engines in which the exhaust flows over a first stage reducing catalyst of tungsten (0.1 to 3 weight percent) and then over a second stage oxidizing catalyst of, e.g., platinum. The two-stages as provided allow the tungsten to utilize HC in the exhaust to first reduce the NO.sub.x and then the resultant exhaust gas is oxidized over platinum to convert the HC and CO. One drawback of this system is that it is less than commercially desirable to use two-stage systems.